1. Field of the Invention
The present invention relates generally to a technique for degassing of molten metal for producing high purity metal products, such as extra low carbon steel. More specifically, the invention relates to a method and apparatus for degassing molten metal utilizing the RH method.
2. Description of the Background Art
Recently, requirements for high quality metal have been substantially increased. Some kinds of metals require vacuum degassing treatment for removing impurities for better quality. To answer such requirement, it is desired to develop a method and apparatus which may allow high efficiency in degassing treatment and lower tapping temperature in a converter for lower production cost.
As is well known, a molten metal in a ladle is introduced into a vacuum chamber for subjecting to vacuum atmosphere to extract and remove carbon and other impurities. For example, Japanese Patent First (unexamined) Publication (Tokkai) Showa 51-71202, published on June 19, 1976, discloses a method of vacuum degassing for molten metal utilizing the apparatus set forth above. The disclosed apparatus has an induction pipe immersed into the molten metal in the ladle for introducing the molten metal into the vacuum chamber from the ladle, and a return pipe to return the molten metal from the vacuum chamber to the ladle. The induction pipe and the return pipe are oriented in spaced apart relationship to each other so that the molten metal in the vacuum chamber flows from the discharge outlet of the induction pipe the returning inlet of the return pipe in the vacuum chamber. By this construction, the molten metal can be maintained for a longer period in the vacuum chamber to remove a greater amount of impurity. Such method is known as the "Ruhrstahl Hausen (RH) method". In such case, the efficiency of removal of impurities is variable depending upon the circulation rate of the molten metal in the vacuum chamber and the surface area of the molten metal in direct contact with the vacuum atmosphere in the vacuum chamber. In the RH method, an inert gas, such as Ar gas, is blown into the induction pipe for circulating the molten metal in the ladle into the degassing chamber. The resulting Ar bubble serves not only for circulating flow of molten metal through the degassing or vacuum chamber but also for providing greater surface area in the vacuum chamber by bubbling at the surface. Therefore, such gas blow technology is well introduced in the RH method for the sake of higher impurity removal efficiency.
As will be naturally appreciated, for increasing the circulation rate of the molten metal, it is important to increase the gas flow rate through the induction pipe. To increase gas circulation rate, it is required to provide a wider path area of the induction pipe communicating between the ladle and the vacuum chamber. However, the path area of the induction pipe is limited by the internal diameter of the vacuum chamber. Therefore, in order to expand the path area, it becomes necessary to expand the diameter of the vacuum chamber, which causes necessity of re-construction of associated facilities.
On the other hand, in the usual degassing process, the gas pressure to be blown into the molten metal is limited to lower than or equal to 9.9 kg/cm.sup.2. Such relatively low pressure gas is blown into the molten metal and introduced into the vacuum chamber via an induction pipe of 3 to 5 mm diameter. In the practical construction, 8 to 20 gas discharge pipes or nozzles are circumferentially arranged around the ladle for blowing the gas. However, because of low pressure, a bubble formed gas blow is formed around the periphery of the induction pipe leaving the central position of the induction pipe not bubbled. Such flow of the bubble in the induction pipe will be hereafter referred to as "peripheral bubble flow". The peripheral bubble flow in the conventional art is not at all effective to improve impurity removal efficiency as required or expected.
In order to improve such drawbacks in the conventional art, Japanese Patent First Publication (Tokkai) Showa 51-6103, published on Jan. 19, 1976, discloses an apparatus which blows the inert gas to an orientation in the vicinity of the center axis of the induction pipe. This is successfully introduced for capability of formation of the bubble at the central portion of the induction pipe thus improving impurity removal efficiency. Such flow of the bubble will be hereafter referred to as "center bubble flow". However, this still has some drawbacks. For instance, since the gas nozzle has to be introduced into the molten metal in the ladle it is subject to substantial heat to shorten the life of the nozzle. Furthermore, since the nozzle is heated at a substantial temperature, replacement of the nozzle for repair becomes difficult and an unacceptably long period. Furthermore, such center bubble type flow tends to cause temperature fluctuation on the peripheries of the vacuum chamber and the induction pipe to cause spalling on the refractory.